Compact information devices that can be carried and used by a user outside the home, for example, a cellular phone such as a smart phone, a tablet terminal, an electronic book, and a portable music player have been widely used. In recent years, information devices used by being mounted on a part of the body, for example, the head or arm of the user have been increased. For example, head-mounted displays and wristband type devices have been used. The head-mounted display is used by being mounted on the head or face of the user. An image display unit is disposed facing each of the left and right eyes (or a single eye). An enlarged virtual image of a displayed image is formed by a virtual image optical system. In this manner, it is possible to present an image with realistic sensation. It is also possible to present AR (Augmented Reality) information superimposed on a field of view of the user.
A compact information device of this type is basically configured to be driven by a storage element such as a secondary battery as a main power source, considering the use in an environment without a power supply outside the home. Moreover, the secondary battery has a limited operational time. Therefore, for charging it anywhere, many users carry a portable charger. In addition, in recent years, development of portable power generation apparatuses that obtain electrical power by power generation using environmental energy or the like anywhere has been progressed. Examples of an energy source in energy harvesting can include environmental electromagnetic waves, sunlight, vibrations, and heat.
If it is an information device used by being mounted on a part of a body of the user, by incorporating the power generation apparatus in the device, power generation can be automatically performed by the user, using human ordinary actions such as walking. For example, a power generation apparatus installed in a shoe worn by a user (e.g., see Patent Documents 1 and 2) uses the weight of the user as external weight, and hence can generate electrical power by walking or running in an ordinary life. Thus, a practical power generation apparatus can be provided. Using the obtained electrical power, the power generation apparatus of this type can drive a portable device or charge a secondary battery.
The user of the power generation apparatus has to check whether or not the secondary battery has a sufficient power storage amount. If the power storage amount is sufficient, the user can detach the storage battery from the power generation apparatus and replace a secondary battery in an information device used by the user with the storage battery or carry the storage battery as a spare battery.
By the way, if the user checks the power storage amount of the secondary battery and the power storage amount is insufficient, the action of checking and replacing the battery becomes in vain. If the information device incorporates the power generation apparatus, the information device can display a charging condition of the battery on a display and inform the user of this. In contrast, if the power generation apparatus is installed outside the information device (e.g., in shoes worn by the user), the information device does not include means for obtaining information on the power storage amount of the secondary battery in the power generation apparatus. Thus, the user should stop the use of the information device and check the power storage amount by viewing the power generation apparatus, which is cumbersome. If the information device is a type worn by the user, for example, a head-mounted display, it can be reasonable to place the power generation apparatus outside instead of incorporating the power generation apparatus in the information device in view of a weight burden on the user.
If the power generation apparatus is installed in each of the left and right shoes worn by the user or if a plurality of power generation apparatuses using energy harvesting or the like are dispersedly arranged in a surrounding environment of the user, checking the power storage amount of each of the power generation apparatuses and judge which of the secondary batteries of the power generation apparatuses should be used is troublesome for the user.